<!doctype html>
<head>
  <style>
    body {
      font-size: 16px;
    }

    ul {
      font-size: 0.8em;
    }

    ul ul {
      font-size: 1em;
    }
  </style>
</head>

<body>
  <ul>
    <li>
      Top level
      <ul>
        <li>
          Second level
          <ul>
            <li>
              Third level
              <ul>
                <li>
                  Fourth level
                  <ul>
                    <li>Fifth level</li>
                  </ul>
                </li>
              </ul>
            </li>
          </ul>
        </li>
      </ul>
    </li>
  </ul>
</body>
